1. Field of the Invention
The present invention relates generally to waveshape monitoring devices, and more particularly to a method and apparatus for comparing electrocardiogram waveshapes.
2. Description of the Prior Art
Waveshape monitoring devices typically detect particular waveshape portions of a more generalized time-varying signal having a variety of waveshapes. Particular use for such devices is found in heart monitoring equipment, such as arrhythmia detectors. One common form of waveshape detection generally used in arrhythmias detectors compares portions of an input waveform to a stored waveform. In this type of comparison, a portion of the input waveform is stored as a template comprising a group of time sequential amplitude values representative of the values of successive amplitudes of the signal portion which is to be detected. The initial signal portion detected and used for forming the template may be visually selected by the operator of the monitoring equipment. Thereafter, successive groups of time-sequential amplitude values of the input waveform are sequentially compared to the stored template values during the monitoring of the input waveform by the monitor. If a correlation greater than a predetermined confidence level is found, a signal is generated which indicates the input waveshape portion matches the stored template.
In the field of arrhythmia monitoring, QRS complexes of an electrocardiogram (ECG) signal are analyzed to determine whether they are atrial or ventricular in origin. A common technique used to classify QRS complexes is to compare them to other QRS complexes stored in templates which have already been classified. Correlation is frequently used to determine the degree of similarity between two QRS complexes. Correlation is performed on a reference complex x1 and a candidate complex x2 according to the following equation: ##EQU1## If the correlation coefficient r is greater than a fixed limit, e.g., 0.98, then the candidate complex is classified the same as the template that is matched.
One of the problems with this technique is that it is computationally demanding because of the number of multiplications and divisions which are required for determining the correlation coefficient. This problem is compounded because correlation is typically performed on many templates before a match can be determined. For example, the monitoring equipment may include not only stored templates for various portions of the ECG signal but also recurrent variations of specific portions, e.g., recurrent variations of the QRS complex. Furthermore, it may be desirable to compare entire QRS complexes including the T wave; however, because correlation requires considerable processing, many systems only use the QRS complex for correlation purposes in order to reduce the amount of required computations. An additional problem with correlation is that it is sensitive to misalignment of the QRS fiducial point.
An object of the present invention is to simplify the waveshape comparing process by reducing the number of samples stored in the template, thereby reducing the number of computations which must be performed. A further object of the invention is to reduce the complexity of the computations which are required to be performed in order to rapidly accomplish a reliable comparison between waveshapes using uncomplicated means.